1. Field of the Invention
The present invention relates to a shadow mask for a Cathode Ray Tube (CRT), and more particularly, to a shadow mask for a CRT which can improve the brightness and the white uniformity and minimize the light emission of incorrect colors.
2. Description of the Related Art
Generally, a Cathode Ray Tube (CRT) is an electronic tube where electron beams emitted from an electron gun are deflected due to a deflection magnetic field, pass through a color selection shadow mask, and then strike and excite green, blue, and red phosphors on a phosphor film within a panel, thereby displaying desired images.
The shadow mask has a color selection function of selecting the emitted electron beams and landing them on the phosphor film. For this purpose, beam passage holes are arranged at the shadow mask in a predetermined pattern to pass the electron beams.
The beam passage holes of the shadow mask are formed in the shape of a circle or a rectangle. When the beam passage holes are formed in the shape of a rectangle, the beam passage holes are arranged such that the long sides of the beam passage holes are parallel to the vertical direction of the shadow mask. The beam passage holes are disposed between bridge portions.
The beam passage holes are formed using a photo etching technique such that the etching is made at both surfaces of the shadow mask. That is, a photoresist is coated onto both surfaces of the shadow mask material, and a pair of disks patterned to correspond to the beam passage holes to be formed are tightly adhered to the photoresist films, followed by exposing the material to light and developing it to form photoresist patterns corresponding to those of the disks. The shadow mask material with the photoresist patterns is etched at both surfaces thereof to thereby form the beam passage holes.
A beam passage hole as described above is formed with a small size on one surface of the shadow mask, and with a large size on the other surface of the shadow mask, and the shadow mask is installed in such a way that the small-sized hole is toward the electron gun and the large-sized hole is toward the panel.
Even if the four corners of the disk which is patterned corresponding to the beam passage holes are formed at an exact right angle to form an exact rectangular pattern of the beam passage hole, the four corners of the shadow mask are formed in the shape of an almost circular arc due to unclear developing of the photoresist pattern, the difference of the etching rates, and other reasons.
Accordingly, the light emission pattern of the phosphors which receive the electron beams is not the exact shape of a rectangle, and the brightness and the white uniformity of the CRT are deteriorated due to the distortion of the pattern.
When the beam passage hole is formed by etching from both of the surfaces of the shadow mask, a boundary portion is formed between the large-sized hole and the small-sized hole. The boundary portion has the shape of a protrusion to the inside of the hole.
Since the electron beams which are emitted from the electron gun pass through the shadow mask almost vertically with respect to the surface of the shadow mask, they land on the phosphors corresponding to the beam passage hole exactly in the middle of the shadow mask.
But in the corner of the shadow mask, the electron beams are deflected so much that parts of the electron beams collide with the boundary portion or the inner surface of the beam passage hole. Accordingly, the electron beams do not land on the phosphors exactly corresponding to the beam passage hole but rather they land on the incorrect phosphors or a black matrix.
Accordingly, light emission of incorrect colors occurs, and the purity of the color and the contrast of the CRT are deteriorated.
In the shadow mask according to the prior art, a bulging portion or a concave portion is formed by extending parts of the beam passage hole toward the bridge portion to solve this problem. This is described in Korean Patent No. KR1992-10719, and Japanese Patent Nos. JP1-175148, JP1-320738, JP55-159545, and JP56-156636.
But if the bulging portion is too large, the shape of the electron beams passing through the beam passage hole also becomes large. Accordingly, not only the phosphors corresponding to the beam passage hole but also incorrect phosphors can emit light.
On the other hand, if the bulging portion is too small, it is hard to prevent the electron beams from colliding with the inner surface of the beam passage hole.
Accordingly, determining the size of the bulging portion properly is extremely important, but there has been no disclosure regarding this factor in the prior art.
Recently, the deflection angle has increased to more than 110 degrees for slim CRTs, and the size of the bulging portion is more important in these CRTs as the deflection angle is increased.